No Love Lost
Mikhail Faustin wants you to find out if his daughter is dating a biker, and if he is, kill him.
Grab a vehicle. As always, I suggest 'borrowing' Faustin's Turismo, then make your way to Firefly Island. Park in the mission objective icon and watch the cutscene. The guy will try to escape on a bike, so jump on the bike conveniently parked there and chase after him.
There's not a lot of point wasting ammo on him yet, as he's very difficult to kill, however I will point out that it is possible to take him out now. It's much easier to kill him later on though.
On His Tail
If you stay on his tail, he'll get a bunch of his biker 'brothers' to join him, and they'll lead you to a park. Head in there and drive by them. Most of them should die fairly easily, but make sure you keep moving to avoid being killed. The main biker will have a machine gun so be careful for that. Try to stay as far away from them as you can, then get off the bike and use auto aim to help you take them out. Using trees as cover is a good tactic too.
You need to kill all of them, then once they're all dead, Niko will call Faustin.
- The main biker will drop an AK-47 after your battle in the park, which is a very useful weapon to have, so don't forget to grab it!